Solutions and Mixtures - AP Chemistry Unit 3, Topic 7

3.7 Mixtures and Concentrations in Solutions

Understanding Mixtures

  • Definition of Mixtures: Materials composed of more than one substance. Examples include:

    • Salad

    • Cheeseburgers

    • Solutions like Kool-Aid or saltwater

Types of Mixtures

Heterogeneous Mixtures

  • Definition: Mixtures where individual components (phases) are visible and distinct.

  • Example: In a salad, lettuce, tomatoes, and cucumbers are all visible phases.

Homogeneous Mixtures

  • Definition: Mixtures where individual components are uniformly mixed and not visibly distinguishable.

  • Example: Dissolving Kool-Aid or salt in water results in a uniform solution where components can't be seen.

  • Term Used: Homogeneous mixtures are commonly referred to as solutions.

Solutions Overview

  • Common Types of Solutions: Typically consists of a solid dissolved in a liquid but can also be mixtures of gases (e.g., air) or solids (e.g., alloys like brass or sterling silver).

  • Focus Area: Primarily discussing solids dissolved in liquids, especially water.

Concentration of Solutions

  • Definition: Concentration quantifies how much solute is dissolved in a solvent.

  • Common Units of Concentration:

    • Molarity (M): Moles of solute per liter of solution (most emphasized in AP Chemistry).

    • Other units include: formality, molality, percent by mass, percent by volume, normality.

Molarity Calculation

  • Equation: [ \text{Molarity} = \frac{\text{moles of solute}}{\text{liters of solution}} ]

  • Example Problem: Calculating the molarity of a solution produced from 0.711g of tin(II) chloride in 75.0 mL of water:

    • Convert grams to moles:

      • Molar mass of tin(II) chloride = 189.62 g/mol ➔ 0.711 g = 0.00375 moles.

    • Convert volume to liters: 75 mL = 0.0750 L.

    • Calculate molarity: [ M = \frac{0.00375\text{ moles}}{0.0750\text{ L}} = 0.0500 , M ]

Solving for Molarity

  • Second Example: Determining molarity of 2.556 g of zinc chloride in 250.0 mL of water:

    • Molar mass calculation:

      • Molar mass of zinc chloride = 136.31 g/mol ➔ 2.556 g = 0.01875 moles.

    • Convert 250 mL to liters: 0.250 L.

    • Calculate molarity: [ M = \frac{0.01875\text{ moles}}{0.250\text{ L}} = 0.0750 , M ]

Working with Molarity

  • Finding Grams from Molarity: If you have volume and molarity and need to find grams:

    • Example: 50.00 mL of 0.250 M NaOH:

      • Convert volume to liters: 50 mL = 0.0500 L.

      • Calculate moles using molarity: [ moles = M \times L = 0.250 , M \times 0.0500 , L = 0.0125 , moles ]

      • Convert to grams: Molar mass of NaOH = 40 g/mol ➔ 0.0125 moles = 0.500 g of NaOH.

Dilution of Concentrated Solutions

  • Common Laboratory Practice: Preparing a less concentrated solution from a more concentrated one.

  • Example: Creating 500.0 mL of 0.250 M HCl from 12.0 M HCl involves:

    • Determining moles needed first from the desired molarity.

    • Using the dilution equation: ( C_1V_1 = C_2V_2 ) to find the volume of concentrated solution required.